One quick-charge battery charger that has been proposed in recent years detects a voltage drop across a battery being charged to control a charging current in a final charging stage after the battery has been charged at a relatively high current rate. More specifically, as shown in FIG. 1 of the accompanying drawings, while a battery (more particularly a nickel-cadmium battery) is being charged by a battery charger, a constant charging current is supplied to the battery, resulting in the voltage across the battery progressively increasing with time. In the final charging stage, the voltage across the nickel-cadmium battery reaches a peak value Vp, and then drops by -.DELTA.V from the peak value Vp. When the voltage drop -.DELTA.V is detected, the charging current is cut off to prevent the nickel-cadmium battery from being excessively charged. After the quick charging current has been cut off, the nickel-cadmium battery is continuously trickle charged with a low current in order to keep the battery fully charged.
Detecting the voltage drop -.DELTA.V is an effective way of determining when the charging of the battery is completed, because the voltage drop -.DELTA.V occurs regardless of the discharge rate or ambient temperature of the nickel-cadmium battery.
It is known that a nickel-cadmium battery which has not been in use (i.e., which has been in storage for a long period of time), has its charging capacity reduced because of the inactive state that develops when the battery is stored for a long time. A battery that has been in storage for a long period of time will hereinafter be referred to as a long storage battery. A battery that has been used frequently will hereinafter be referred to as a frequently used battery.
When a long storage battery is charged, a voltage peak called a pseudo voltage peak, is developed prior to the voltage peak, at completion of the charging process. A voltage drop immediately following such a pseudo actual voltage peak at the completion of the charging of the battery. If the charging current supplied to charge a long storage battery is cut off when the voltage drop subsequent to such a pseudo voltage peak is detected, the battery would not be fully charged.